Curious what oil viscosity you run. I have always used the Valvoline 20W50 race oil. I looked on the 351C Facebook group site and the references suggest pretty high pressures on the motor.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.